In the manufacture of flat bed trailers for use in hauling loads over roadways, such trailers typically include at least a pair of elongated support beams and perhaps more than two support beams which serve as the primary load supporting elements of the trailer construction. In the design of such beams, it is typically appropriate to construct the beams of generally "I" cross section, with the end sections of the beams having substantially lesser depth than the intermediate section thereof.
A typical flat bed trailer beam, for example, may be 44 feet in length and may be constructed from steel, such as A-441 grade low alloy steel. The upper flange of the beam may be composed of steel plate that is 5/8 inch in thickness by 6 inches in width. The web of the beam construction may be composed of steel plate in the order of 3/16 inch in thickness, while the lower flange of the beam may be composed of steel plate in the order of 3/8 inch in thickness by 6 inches in width. The web of the beam may be reduced to a width of 8 inches in the end sections while the central portion of the web may be at its full width of 22 inches. The transition from the 22 inch width of the central portion of the beam to the 8 inch width of the end sections is defined by a 92 inch curved transitioning section of the beam construction, the curves at the transition points being defined in approximately 20 inches in length.
The lower flange of the beam construction is typically attached to the web of the beam before the upper flange is welded to the web portion of the beam construction. Welding the lower flange to the beam, as an initial step, causes approximately 4 inches of camber to develop in the beam because of the stresses that develop during the welding process. After welding the upper flange to the web portion of the beam, a portion of the camber is removed by counterbalancing welding stresses. With the completed beam in its operative position, approximately 11/2 inches of camber will remain, thereby causing the centermost portion of the beam construction to be 11/2 inches above the level of the extremities of the beam. This amount of camber is desirable because of the deflection that occurs when the trailer assembly is fully loaded. For example, when a trailer of 44 feet in length is loaded with its maximum weight of 45,000 pounds of evenly distributed load, approximately 21/2 inches of deflection will occur in the longitudinal beams of the trailer construction. This amount of deflection is within optimum design perimeters.
In the plant production of truck trailer beams such as those described above, it has been determined that the hand welding process adds materially to the cost of the beams and can render fabricated trailer beams excessively expensive because of the labor costs involved and because of the shop time necessary for hand welding of the beam constructions. It is desirable therefore to provide an automatic and substantially labor free welding process for the beams. Template controlled and sensor computer controlled welding devices are available for automatic arc welding and simple beam carriage type apparatus is available for welding in a straight line and on relatively flat work pieces, but these are undesirable for the various reasons explained below. The cost of template controlled or sensor computer controlled devices for accomplishing automatic welding of truck trailer beam constructions is considered excessively expensive for the volume of beam production that is typically required at a truck trailer manufacturing facility. Additionally, template controlled welding or sensor computer controlled welding of truck trailer beams necessitates design changes in the beam construction to accommodate the welding process and detracts from the design feasilibity of the beams that are required for optimum load supporting characteristics.
Simple beam carriage type welding apparatus, that may be efficiently employed for making flat welds on flat work pieces cannot be utilized with any degree of efficiency or accuracy for welding along the curved joint that is established when the flanges of the beam construction are welded to the web portion of the beam. The simple joint tracking apparatus of a beam carriage type welding mechanism will not precisely follow the curve that is established when a curved flat bar or sheet is joined to a flat plate with a curved edge because the curve, when viewed from the 45.degree. angle of the electrode accomplishing welding of a flange to the curved edge of the web, does not lie in a single plane. Actually, the curve is of compound nature, thereby requiring that the curve tracking apparatus be capable of substantial universal movement in order to properly track both the concave and convex curved sections of the beam.
It is also desirable to accomplish both fillet welds on either side of the web at the same time in order to eliminate any transverse bending or distortion that would otherwise occur if the welds were accomplished on one side only during a single pass of the welding apparatus. It is appropriate, therefore, to provide a pair of welding heads that are simultaneously operative to accomplish fillet welding on both sides of the web. In this case, each welding head must be capable of precisely tracking the compound curve that is encountered in order to cause the electrode support portion of the welding head to be precisely directed at the joint between the flange and the web at all times.
One of the problems that occurs when a simple beam carriage type welding apparatus is employed in an attempt to accomplish fillet welding of the compound curved joint between the web and flanges of a flat bed trailer beam construction is the tendency of the tracking rollers to climb the vertical surface of the flange due to the frictional engagement between the roller and the flange until the frictional engagement is overcome by forces acting downwardly on the welding head support mechanism. When the climbing force is overcome, the tracking roller will slip downwardly to its proper position relative to the joint between the flange and the result will be a discontinuity of the fillet weld that is being accomplished. When this occurs, the fillet weld will track away from its proper position to the joint between the flange and the web causing the weld being formed to also depart from its proper relation with the joint and, upon slipping, will cause increments of discontinuity to occur in the fillet welds being formed. This is very undesirable and results in the formation of a weak as well as aesthetically displeasing weld between the flange and the web portions of the beam construction. It is desirable, therefore, to provide a beam carriage type welding mechanism that is specifically constructed in such manner that the electrode support portion of the welding head will precisely track the joint between the flange and the web at all times and will not allow any slipping or jumping of the guide roller to occur which would otherwise cause the development of weld discontinuity and inaccuracy.